Many Radio Frequency (RF) transmitters and receivers use In-phase/Quadrature (I/Q) configurations, in which a signal is represented by two orthogonal signal components. In some cases, however, the transmitter and/or receiver circuitry introduce I/Q imbalance impairments, which distort the signal and degrade the reception performance. Reception performance may also be degraded by frequency offsets introduced by the transmitter and/or receiver.
Various techniques for compensating for I/Q imbalance and frequency offset impairments are known in the art. For example, U.S. Pat. No. 7,433,298, whose disclosure is incorporated herein by reference, describes techniques for correcting residual frequency offset, phase noise and I/Q imbalance in Orthogonal Frequency Division Multiplexing (OFDM) communications. The disclosed techniques compensate for I/Q imbalance based on most likely estimates of the I/Q imbalance between the I and Q components of a baseband signal. Most likely estimates of the common phase error are sometimes used to compensate for the initial channel estimates.
PCT International Publication WO 2009/082457, whose disclosure is incorporated herein by reference, describes a signal processing method and system for compensation of I/Q imbalance for a direct conversion packet-switched OFDM communications system. Both frequency-dependent I/Q imbalance and frequency-independent cases are considered, covering both wideband and narrowband modulation.
U.S. Pat. No. 7,181,205, whose disclosure is incorporated herein by reference, describes a self-calibration procedure that corrects the amplitude and phase errors of the modulation and demodulation in a transceiver. A test signal is applied to the baseband input of the transmitter, and the output of the modulator is looped back to the input of the demodulator. The amplitude and phase errors of the resulting signal at the baseband output of the receiver are detected, and the contributions of the transmitter and receiver to the errors are separated and resolved into amplitude and phase components. Adjustments are then made to the amplitude and phase balance in the transmit and receive signal paths to correct the errors.